High density, viscosified, aqueous compositions having superior stability under stress conditions

ABSTRACT

Advantageous aqueous mixed salt systems viscosified with water-soluble or water-dispersable polymers which are superior to corresponding single salt systems of similar densities are provided. The mixed salt systems comprise water; a water-soluble or water-dispersable polymer capable of viscosifying an aqueous medium; one or more cations including a member selected from the group consisting of lithium, sodium, potassium, cesium, magnesium, calcium, zinc, or mixtures thereof; and one or more anions including a member selected from the group consisting of chloride, bromide, iodide, formate, nitrate, acetate, cyanate, thiocyanate, a zinc complex anion or mixtures thereof; there being present either at least two cations or at least two anions. Inventive viscosified mixed salt systems display--under such stressing factors as aging, heat, mechanical agitation, and shear--greater stability compared to the single salt systems of similar densities. Also provided are methods for making viscosified mixed salt systems and methods for advantageously using the same as drilling, drill-in, completion, hydraulic fracturing, work-over, packer, well treating, testing, spacer, or hole abandonment fluids.

REFERENCE TO RELATED APPLICATIONS

This application claims priority upon U.S. patent application Ser. No.60/010,051 filed Jan. 16, 1996, which is hereby incorporated byreference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to the exploitation ofsubterranean formations. More specifically, it relates to theexploitation of subterranean petroliferous formations using high densitymixed-brine-based fluids such as drilling, drill-in, completion,hydraulic fracturing, work-over, packer, well treating, testing, spacer,or hole abandonment fluids. Yet more specifically, the field of thisinvention is fluid rheology, thickeners, viscosifiers, viscoelasticfluids, and the physical hydration of polymer additives into highdensity mixed-brine-based drilling, drill-in, completion, hydraulicfracturing, work-over, packer, well treating, testing, spacer, or holeabandonment fluids.

2. Discussion of Related Art

A wide variety of methods for exploiting subterranean petroliferousformations are known in the art, and the problems associated therewithare also well known. Brines are commonly used in drilling, drill-in,work-over, hydraulic fracturing, completion, packer, well treating,testing, spacer, or hole abandonment fluids because of their widedensity range and their character of being free of suspended solids andessentially non-damaging to subterranean petroliferous formations.During the process of drilling and completing an oil well, it is oftendesirable to add polymer and possibly bridging agents, to viscosify thedrilling or completion fluid and thereby to control fluid loss to theformation. As fluids are lost into the formation, these materials filterout and build up a filter cake at the rock face which limits furtherflow of fluids into the formation. Some fluids nevertheless invariablyflow into the formation and can interact with formation matrix in such away as to reduce the permeability of the formation to the subsequentin-flow or out-flow of fluid, especially oil, gas, condensate or otherfluid targeted for withdraw and use. This reduction in the rockpermeability is termed "formation damage".

Xanthan gum is commonly used as a viscosifying polymer in brine-baseddrilling fluids whereas the cheaper hydroxyethyl cellulose (HEC) iscommonly used in a work-over fluid. Xanthan gum has superior toleranceto high pH and temperature and has superior thixotropic propertiescompared to other viscosifying polymers. The viscosifying polymer isusually added to the brine to thicken it so that it will have, forexample, high carrying capacity for the cuttings produced while drillingand high viscosity for a work-over fluid to control fluid loss andminimize formation damage. Xanthan gum also has the ability to impartgel character to a brine so that it will have high carrying capacity fordrill cuttings even when the drilling process is interrupted and thefluid becomes quiescent.

HEC is a typical viscosifier and fluid loss control agent which is knownto cause relatively little damage to the formation. Guar gum and starchderivatives can also be used. However, HEC and other polymers are veryslow to viscosify brines having densities above about 12.0 ppg and HECdoes not viscosify formate brines. Heating can be required to reach adesired viscosity for some brines.

For many applications of brine-based drilling fluids, HEC lackssufficient thermal stability and carrying capacity for the drillcuttings. In these cases, therefore, xanthan gum is typically usedinstead. While such agents as HEC and xanthan gum impart both viscosityand fluid loss control to the drilling, drill in, completion, hydraulicfracturing, work over, well treating, spacer, or hole abandonmentfluids, starches are often added to augment the fluid loss properties.Standard brine based drilling fluid may also include a bridging agent,such as, for example, sized particles of calcium carbonate or sodiumchloride. In addition, a representative brine based drilling fluid canalso include, for example, corrosion inhibitors, lubricants, pH controladditives, surfactants, solvents, and/or weighting agents.

Conventional techniques for viscosifying a brine are limited by the factthat so much of the water in the brine is devoted to solubilization ofthe salt that there is not enough water left over for the solubilizationof the viscosifying agent. In effect, the viscosifying agent is "saltedout". Additionally, mixed-salt brines are often limited by saltsolubilities to relatively low densities. For example, when a densebrine based on NaBr is added to a dense brine based on Ca(NO₃)₂, theprecipitation of solid NaNO₃ depletes the solution so much that theremaining brine is only of relatively moderate density. As anotherexample, the same sort of interaction and precipitation occurs when adense brine based on CaCl₂ or CaBr₂ is added to a dense brine based onK₂ CO₃. The present invention teaches a group of mixed salts which arepreferred because they can be formulated up to relatively high densitiesand yet the availability of "free" water is sufficient to allowviscosifying polymers to hydrate acceptably up to relatively highconcentrations. Hence, inventive mixed-salt compositions are capable ofexhibiting relatively high viscosities.

Problems occur when attempting to use xanthan gum to viscosify highconcentrations of divalent-cation-based brines where most of the wateris associated with salt. These problems include the need for extendedmixing times, high shear, and/or heat in order to viscosify the brine.Similar problems occur with other viscosifiers. These problems arerelatively minor in the brine concentration range extending most of theway from fresh water to almost saturated brine, then suddenly theproblems get much more serious when just a relatively small amount ofextra salt is added to a near-saturated brine.

One example is viscosifying xanthan gum in brines having highconcentrations of CaBr₂, where most of the water is occupied by thesalt. This system requires extended mixing times, high shear, and/orheat in order to viscosity the brine. (See Table 1A).

                                      TABLE 1A                                    __________________________________________________________________________                 Base Brine                                                                           Steps Needed                                              wt. %  %     Density                                                                              to Viscosify                                              CaBr.sub.2                                                                           saturation                                                                          (ppg)  the Brine Uniformly @ 1-5 ppb                             __________________________________________________________________________    0 to 47.9                                                                            0.0 to 83.5                                                                         8.3 to 13.5                                                                          Stirring xanthan gum into the brine at room                                   temperature; no extra shear needed; no extra                                  heating needed.                                           48.5 to 51.3                                                                         84.5 to                                                                             13.6 to 14.1                                                                         Stirring xanthan gum into the brine at room                      89.4         temperature followed by moderate shearing                                     or mild warming.                                          51.9   90.4  14.2   Stirring xanthan gum into the brine followed                                  by extensive shearing at room temperature or                                  mild shearing combined with moderate                                          heating.                                                  52.4 to 57.4                                                                         91.3 to                                                                             14.3 to 15.35                                                                        Stirring xanthan gum into the brine at room                      100.0        temperature followed by extensive mixing                                      and shearing while heating steadily until                                     polymer hydrates sufficiently.                            __________________________________________________________________________

The data clearly show the need for extended mixing times, high shear,and/or heat in order to viscosify CaBr₂ brines above about 85%saturation.

When 3 pounds per barrel (ppb) of xanthan gum is added to a 14.2 poundsper gallon (ppg) substantially pure CaBr₂ solution, stirred vigorouslyfor an hour at room temperature, 70° F., and sheared for 10 minutes on aSilverson mixer Model L4RT at 4000 rpm, it does not viscosity byindication of a yield point (YP) of 0 measured on a variable speedrheometer. It was noticed that by stirring the solution energeticallyfor one hour at 125° F. the xanthan gum viscosified with a YP of 62 anda viscosity at the 3 rpm reading of 2700 cp as shown in Table 1B.Stirring shears the solution but not nearly the amount of shear theSilverson mixer can provide.

To determine the temperature at a specific shear needed to viscosify14.2 ppg CaBr₂ solutions, a Fann Model 50 study was performed to monitorthe viscosity at shear and temperature with time. The solutions wereslowly heated to 150° F. at 0.5 degrees per minute and allowed to remainat that temperature until maximum viscosity was achieved. Two separatetests were performed at shear rates of 511 sec⁻¹ (300 rpm) and 170 sec⁻¹(100 rpm). The temperatures at which 25% of maximum viscosity wasobtained were found to be 122° F. and 143° F., respectively. FIG. 1illustrates these results and shows that 2.5 additional hours are neededto viscosify the solution at 170 sec⁻¹ (100 rpm) than at 511 sec⁻¹ (300rpm). Table 1B below also shows the maximum viscosity of this 14.2 ppgCaBr₂ solution obtained by different methods. This indicates thatheating and shearing equipment would be needed in order to fullyviscosity this solution.

                                      TABLE 1B                                    __________________________________________________________________________    14.2 ppg CaBr.sub.2 + 3 ppb of Xanthan                                            Viscosity, cp                                                                         Viscosity, cp                                                                          Viscosity, cp                                                                          Viscosity, cp                                   RPM 70° F.                                                                         70° F.                                                                          70° F.                                                                          70° F.                                   __________________________________________________________________________    600 11      78       81       84                                              300 11      109      109      107                                             3           2700     2600     2400                                            PV  11      47       53       61                                              YP  0       62       56       46                                              n           0.32     0.33     0.33                                            K, cp       7000     6700     6100                                            Method                                                                            Stirred 1 hour at                                                                     Stirred energetically                                                                  Fann Model 50 at                                                                       Fann Model 50 at                                    70° F., shear 10                                                               for 1 hour at 125° F.                                                           170 sec.sup.-1 at 150° F.                                                       511 sec.sup.-1 at 150° F.                    min/4000 rpm     for 5 hrs.                                                                             for 2 hrs.                                      __________________________________________________________________________

It has been shown that biopolymers like xanthan gum show a transitiontemperature (T_(m)) in brines of various densities. A transitiontemperature is the temperature at which the polymer undergoes anorder-disorder conformation change. This conformation change isaccompanied by a massive loss of viscosity and increase in the rate ofhydrolytic degradation by two orders of magnitude. It has also beenshown that CaBr₂ solutions above 10.4 ppg have a T_(m) of less than 80°C. and that degradation occurs at higher densities. T_(m) has been usedas a guide for predicting thermal stability of the polymers in brinesolutions.

A variety of well servicing fluids and associated systems have beenproposed in the prior art. There has remained a need for improvedsystems having advantageous characteristics for viscosification andfluid loss control, dispersability and hydration. Accordingly, thepresent invention is directed toward enhancing the thermal stability,viscosity and gel structure of dense brine-based drilling, drill-in,completion, hydraulic fracturing, work-over, packer, well treating,testing, spacer, or hole abandonment fluids and toward increasing thethermal stability of the water-soluble or water-dispersable polymer usedto viscosity and gel the brines.

SUMMARY OF THE INVENTION

According to the present invention, there are provided mixed saltcompositions that are viscosified easily and have superior stabilitycompared to a corresponding single salt composition having a similardensity. For example, the present inventors have discovered that a 14.2ppg CaBr₂ and CaCl₂ mixture is extremely difficult or impossible tofully viscosify with xanthan gum or other viscosifiers withoutintroducing significant heat and shear; however, if a 13.6 ppg CaBr₂solution is weighted with dry NaBr to 14.2 ppg it is relatively easierto viscosity without the introduction of significant shear or heat. Thepresent inventors have also discovered, for example, that the stabilityof biopolymers in a calcium-based brine is dramatically improved whenthe formulation of the base brine is optimized with respect to thecalcium chloride and calcium bromide content.

According to the present invention there are provided high-density,viscosified, aqueous compositions having superior stability under stressfactors such as aging, heat, mechanical agitation and shear. Inventivecompositions comprise water; a water-soluble or water-dispersablepolymer; one or more cation selected from the group consisting oflithium, sodium, potassium, cesium, magnesium, calcium, zinc, ormixtures thereof; and one or more anion selected from the groupconsisting of chloride, bromide, iodide, formate, nitrate, acetate,cyanate, thiocyanate, a zinc complex anion, or mixtures thereof; whereinthere are present either at least two cations or at least two anions ineffective amounts to provide advantageous stability. In alternatepreferred aspects of the invention, inventive compositions have superiorheat stability, exhibited by a τ₅₀ of at least about 1; superior shearstability, exhibited either by a ζ₅₀ of at least about 1 or by a ω₅₀ ofat least about 1; superior fluid loss control, exhibited by a ψ₅₀ of atmost about 1; or superior pH stability, exhibited by a Φ of at leastabout 1.

Also provided by the present invention are methods for makinghigh-density, viscosified, aqueous compositions having superiorstability under stress factors. In a preferred aspect of the invention,the method comprises providing an aqueous solution comprising a firstamount of one or more cation selected from the group consisting oflithium, sodium, potassium, cesium, magnesium, calcium, zinc, ormixtures thereof and a second amount of one or more anion selected fromthe group consisting of chloride, bromide, iodide, formate, nitrate,acetate, cyanate, thiocyanate, a zinc complex anion, or mixturesthereof, wherein there are present either at least two cations or atleast two anions in effective amounts to provide the advantageousviscosification stability; and mixing a water-soluble or waterdispersable polymer into the solution to yield a final compositionhaving a τ₅₀ of at least about 1; a ζ₅₀ of at least about 1; a ω₅₀ of atleast about 1; a ψ₅₀ of at most about 1; or a Φ of at least about 1.Preferably, the aqueous solution is prepared by dissolving into water atleast two salts, as described herein, to provide a salt solution havinga density of greater than about 9.5 pounds per gallon (ppg).Additionally provided are methods for using inventive high-density,viscosified, aqueous compositions including using the compositions asdrilling, drill-in, completion, hydraulic fracturing, work-over, packer,well treating, testing, spacer, or hole abandonment fluids.

It is an object of the present invention to provide mixed salt systemsthat are viscosified easily compared to a single salt system havingsimilar density.

Additionally, it is an object of the present invention to provide aviscosified mixed salt system that displays greater stability under suchstressing factors as aging, heat, mechanical agitation, and shear,compared to the single salt system of similar density.

Another object of the present invention is to provide high densitymixed-salt brines having more stable rheological properties, especiallyat elevated temperatures and over extended periods of time, and whichrheological properties are more resistant to heat- andshear-degradation.

It is also an object of the invention to provide viscosified highdensity mixed-salt brines having low crystallization temperatures andbroad ranging compatibility with, for example, fluid-loss agents, seawater, formation water and spacers.

Additional objects, features and advantages will be apparent from thefollowing description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plot of viscosity and temperature versus time and displaysthe temperature at a specific shear needed to viscosify 14.2 ppg CaBr₂solutions.

FIG. 2 is a plot of percent viscosity versus temperature for two singlesalt systems and one mixed salt system.

FIG. 3 is a plot of T₅₀ (° F.) versus percent CaBr₂ in a 10.5 ppg CaCl₂/CaBr₂ solution.

FIG. 4 is a plot of percent viscosity versus temperature, showingviscosity data and the T_(m) for 12.5 ppg NaBr and mixtures of 13.0 ppgCaBr₂ /NaBr and showing viscosity data for pure CaBr₂.

FIG. 5 is a plot of T₅₀ (° F.) versus pounds of NaBr per barrel for a 13ppg CaBr₂ /NaBr solution.

FIG. 6 is a plot of percent viscosity versus temperature for a test ofthermal stability of pre-gelatinized starch as described more fully inExample 5.

FIG. 7 is a plot of T₅₀ (° F.) versus pounds of LiBr per barrel for a12.5 ppg NaBr/LiBr solution.

FIG. 8 is a plot of T₅₀ (° F.) versus pounds of LiBr per barrel for a 13ppg CaBr₂ /LiBr solution.

FIG. 9 is a plot of T_(m) (° F.) versus percent 11.0 ppg NaNO₃ brine in11.0 ppg NaHCO₂ /NaNO₃ brine mixtures.

FIG. 10 is a plot of Yield Point (YP) versus pounds MgCl₂ in a 10.5 ppgCaCl₂ /MgCl₂ solution.

FIG. 11 is a plot of pH versus the log of the molar ratio of totalhalides to zinc, and is described in greater detail in Example 16.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

For the purpose of promoting an understanding of the principles of theinvention, reference will now be made to preferred embodiments thereofand specific language will be used to describe the same. It willnevertheless be understood that no limitation of the scope of theinvention is thereby intended, such alterations and furthermodifications in the invention, and such further applications of theprinciples of the invention as described therein being contemplated aswould normally occur to one skilled in the art to which the inventionrelates.

The present invention provides high density, viscosified, aqueouscompositions having superior stability under stress factors such asaging, heat, mechanical agitation and shear. To advantageously practicethe present invention, starting materials include at least two differentsalts, each comprising a cation selected from the group consisting oflithium, sodium, potassium, cesium, magnesium, calcium, zinc, ormixtures thereof; and an anion selected from the group consisting ofchloride, bromide, iodide, formate, nitrate, acetate, cyanate,thiocyanate, a zinc complex anion, or mixtures thereof. Saltsadvantageously used in accordance with the present invention are readilyavailable commercially.

In one preferred composition according to the invention, there arepresent a cation having a +2 charge and a cation having a +1 charge. Inan alternate preferred aspect of the invention the first cation and thesecond cation each have a +2 charge. In another preferred aspect of theinvention, the composition comprises a chloride anion and a bromideanion. Examples of preferred salt combinations useful for preparinginventive compositions include the following:

lithium chloride/lithium bromide

calcium chloride/calcium bromide

sodium bromide/calcium bromide

lithium bromide/sodium bromide

lithium bromide/calcium bromide

sodium chloride/potassium chloride

sodium chloride/calcium bromide

sodium chloride/sodium bromide

sodium bromide/potassium bromide

sodium formate/sodium nitrate

sodium formate/potassium nitrate

potassium formate/sodium nitrate

sodium formate/cesium nitrate

cesium formate/sodium nitrate

potassium formate/cesium nitrate

cesium formate/potassium nitrate

lithium formate/cesium nitrate

cesium formate/lithium nitrate

potassium formate/calcium nitrate

calcium formate/potassium nitrate

potassium formate/potassium nitrate

cesium formate/cesium nitrate

potassium chloride/potassium bromide

potassium bromide/calcium bromide

potassium chloride/calcium bromide

potassium formate/calcium bromide

magnesium chloride/calcium chloride

magnesium chloride/calcium bromide

magnesium bromide/calcium bromide

magnesium chloride/magnesium bromide

cesium chloride/cesium bromide

cesium chloride/sodium chloride

cesium bromide/sodium chloride

cesium chloride/sodium bromide

cesium bromide/sodium bromide

cesium chloride/potassium chloride

cesium bromide/potassium chloride

cesium chloride/potassium bromide

cesium bromide/potassium bromide

cesium chloride/calcium chloride

cesium bromide/calcium chloride

cesium chloride/calcium bromide

cesium bromide/calcium bromide

calcium chloride/calcium formate

calcium bromide/calcium nitrate

calcium bromide/cesium bromide

zinc complex salts/alkali metal or alkaline earth metal salts

More detailed descriptions of various selected compositions prepared inaccordance with the present invention appear below in the Examples. Itshould be noted that in inventive compositions, the salts aresubstantially dissolved into water. As such, a statement that aparticular inventive composition includes two salts is intended to meanthat the composition includes the cations and anions of the salt,although these cations and anions may be, for example, in the ionicstate or complexed with other ions or materials.

It is also contemplated by the present invention that preferredcompositions may be prepared using as starting materials three or moredifferent salts. In this regard, in one preferred aspect, inventivecompositions comprise three different cations, a first cation having a+2 charge, a second cation having a +2 charge, or a second cation inwhich the cation is held as a complex having cationic, neutral, oranionic form, and a third cation having a +1 charge.

Another starting material for the practice of the invention is awater-soluble or water-dispersable polymer. Preferably the polymer usedin accordance with the present invention is a biopolymer, and is morepreferably selected from the group consisting of algin; anioniccellulose; anionic polysaccharide; cationic polysaccharide;carboxymethyl cellulose; carboxymethyl hydroxyethyl cellulose; gellangum; guar gum; gum ghatti; gum karaya; gum tragacanth; gum arabic; gumacacia; locust bean gum; methacrylic acid polymer; polyamine;polyanionic cellulose; iota, lambda or kappa sulfated polysaccharides;polysaccharides modified by i) cross-linking, ii) hydroxyethylation,iii) hydroxypropylation, iv) carboxymethyl-hydroxyethylation, v)carboxymethylhydroxypropylation, vi)carboxymethylation, or vii)carboxylation; rhamsan gum; vinyl compound polymer; wellan gum orglycol-compatible wellan gum; xanthan or xanthan gum; or mixtures ofsaid polymers. More preferably, the polymer is selected from the groupconsisting of anionic polysaccharide; cationic polysaccharide; gellangum; guar gum; rhamsan gum; wellan gum; xanthan or xanthan gum ormodifications of said polymers by I) cross-linking, ii)hydroxyethylation, iii) hydroxypropylation, iv)carboxymethyl-hydroxyethylation, v) carboxymethyl-hydroxypropylation,vi)carboxymethylation, or vii) carboxylation; or mixtures of saidpolymers. It is readily understood that additional polymers havingthixotropic properties or gel structure properties similar to thosespecifically mentioned may also be advantageously used in accordancewith the present invention. Polymers useful according to the presentinvention are readily available commercially.

Inventive compositions are, generally, dense salt solutions viscosifiedwith a polymer. Inventive compositions comprise water; a water-solubleor water-dispersable polymer; one or more cation selected from the groupconsisting of lithium, sodium, potassium, cesium, magnesium, calcium,zinc, or mixtures thereof; and one or more anion selected from the groupconsisting of chloride, bromide, iodide, formate, nitrate, acetate,cyanate, thiocyanate, a zinc complex anion or mixtures thereof; whereinthere are present either at least two cations or at least two anions ineffective amounts to impart to the composition advantageous stabilityunder stressing factors such as heat, shear and pH.

Inventive compositions are preferably prepared from the followingstarting materials in the proportions given: from about 10.0 to about90.0 weight percent water; from about 0.01 to about 45.0 weight percentpolymer; from about 0.05 to about 85.0 weight percent of a first salt;and from about 0.05 to about 85.0 weight percent of a second salt. In analternate preferred embodiment, also added is from about 0.05 to about85.0 weight percent of a third salt. More preferably, inventivecompositions are prepared using the following proportions of startingmaterials: from about 0.5 to about 10 weight percent polymer; from about2 to about 80 weight percent of a first salt; and from about 2 to about40 weight percent of a second salt and optionally from about 2 to about80 percent of a third salt. Additionally, the relative amounts of eachsalt may advantageously be optimized with minimal experimentation toaccomplish the desired results described herein.

It is readily understood by one of ordinary skill in the art thatadditional materials may also be included in preferred compositions.Examples of such additives include starch, bridging agents (such as, forexample, sized particles of calcium carbonate or sodium chloride),corrosion inhibitors, lubricants, pH control additives, surfactantsand/or weighting agents.

In another aspect of the invention, high-density, viscous compositionsare made by first providing an aqueous solution having dissolved thereinat least one cation selected from the group consisting of lithium,sodium, potassium, cesium, magnesium, calcium, zinc, or mixturesthereof, and at least one anion selected from the group consisting ofchloride, bromide, iodide, formate, nitrate, acetate, cyanate,thiocyanate, a zinc complex anion, or mixtures thereof, wherein there ispresent either at least two cations or at least two anions in amountseffective to achieve the desired stability; and then mixing awater-soluble or water-dispersable polymer into the brine. The aqueoussolution may be provided, for example, by dissolving into water a firstsalt and a second salt, each comprising a cation selected from the groupconsisting of lithium, sodium, potassium, cesium, magnesium, calcium,zinc, or mixtures thereof; and an anion selected from the groupconsisting of chloride, bromide, iodide, formate, nitrate, acetate,cyanate, thiocyanate, a zinc complex anion, or mixtures thereof.Preferably the resulting salt solution has a density of greater thanabout 9.5 pounds per gallon (ppg), more preferably greater than about 10ppg, and yet more preferably greater than about 11.5 ppg.

Alternatively, the polymer is added to a brine, having dissolved thereinone salt, before adding the second salt either in the form of a brine ora solid salt. The resulting composition in either instance preferablyincludes at least about 0.1 pounds per barrel (ppb) of the polymer, atleast about 1% cations by weight, and at least about 1% anions byweight; more preferably at least about 0.2 ppb polymer, at least about5% cations by weight, and at least about 5% anions by weight; and mostpreferably at least about 0.4 ppb polymer, at least about 10% cations byweight, and at least about 10% anions by weight.

Depending upon the specific salt composition of a given system, theaddition of the polymer may be facilitated by heating the mixture whilestirring. An advantageous feature of the invention, however, is therelative ease with which polymers may be hydrated by relatively densesalt solutions. This feature is described in greater detail below in theExamples with regard to various selected inventive compositions. In onepreferred aspect of the invention, the polymer is mixed into the brineby first hydrating, solvating or swelling the polymer at least partiallyin a fluid medium, and then mixing the fluid medium with the brine.

Compositions of the present invention exhibit superior stability undersuch stressing factors as aging, heat, mechanical agitation and shear,this feature being very important in fluids utilized in the field ofsubterranean drilling. The term "stability" as used herein is intendedto refer to the ability of an inventive composition to withstand variousstress conditions before the viscosifying polymer's capability toviscosifying the fluid composition is substantially diminished. By"substantially diminished" it is meant diminished to an extent where thecomposition may no longer be advantageously used for its intendedpurpose. Diminished viscosity is commonly referred to in the relevanttrade as fluid degradation.

A technique used to measure relative heat stability involves adetermination of τ₅₀ of a mixed salt/polymer composition. To determineτ₅₀ of an inventive composition, T₅₀ of a mixed salt composition isdetermined and related to the T₅₀ of one or more of the related singlesalt systems according to the following equation: ##EQU1## wherein (A/B)indicates that the preceding value is a property of the two salt system;(A) represents a corresponding single salt system; and (B) representsthe other corresponding single salt system. The term "correspondingsingle salt system" as used herein is intended to designate acomposition consisting essentially of only cations and anions derivedfrom one salt, this salt being one of the salts dissolved in thepreparation of the particular inventive two or three salt composition.T₅₀ of a composition is determined by heating the composition at a givenrate and recording the temperature at which the viscosity of thecomposition is half of its viscosity at room temperature. In one aspectof the invention, inventive two salt systems exhibit a τ₅₀ of greaterthan about 1. This means that the T₅₀ of the inventive two salt systemis greater than expected based upon the linear relationship between T₅₀values for the two corresponding single salt systems. As such, there aretwo corresponding single salt systems for each inventive two saltsystem. Table 1C shows τ₅₀ computations for various systems according tothe present invention.

                                      TABLE 1C                                    __________________________________________________________________________    τ.sub.50 of Various Mixed Salt Systems                                    Density  (ppg)                                                                    Mixed Salt  System A/B                                                                  ##STR1##  T.sub.50 (A/B)  (F.°)                                                        T.sub.50 (A)  (F.°)                                                         T.sub.50 (B)  (F.°)                 __________________________________________________________________________    12.5                                                                              LiBr/NaBr                                                                              1.084      219.5 114  291                                            (50/50)                                                                   12.5                                                                              LiBr/NaBr                                                                              1.296      262.5 114  291                                            (25/75)                                                                   13.0                                                                              LiBr/CaBr.sub.2                                                                        1.780      266   114  184.8                                      9.0 NaCl/NaBr                                                                              >1.0       300   300  >291                                       10.5                                                                              NaCl/NaHCO.sub.2                                                                       1.173      352   300  300                                        9.0 NaCl/KCl 1.000      300   300  300                                        10.5                                                                              MgCl.sub.2 /CaBr.sub.2                                                                 1.041      255   268  222                                            (35.6/64.4)                                                               10.5                                                                              CaCl.sub.2 /CaBr.sub.2                                                                 1.117      257   238  222                                        12.5                                                                              NaBr/KHCO.sub.2                                                                        0.842      294   290.7                                                                              408                                        10.5                                                                              MgCl.sub.2 /CaCl.sub.2                                                                 0.949      240   268  238                                        10.5                                                                              MgCl.sub.2 /CaBr.sub.2                                                                 0.992      243   268  222                                            (50/50)                                                                   10.5                                                                              MgCl.sub.2 /CaBr.sub.2                                                                 0.988      242   268  222                                            (45/55)                                                                   10.5                                                                              MgCl.sub.2 /CaBr.sub.2                                                                 0.986      241.6 268  222                                            (30/70)                                                                   10.5                                                                              MgCl.sub.2 /CaBr.sub.2                                                                 0.967      237   268  222                                            (25/75)                                                                   13.0                                                                              CaBr.sub.2 /Ca(NO.sub.3).sub.2                                                         0.935      202.5 184.8                                                                              248.3                                      __________________________________________________________________________

Although various systems have τ₅₀ values slightly less than 1, thesesystems have improved characteristics according to various other aspectsof the invention, such as, for example, improved shear stability,improved pH stability and/or improved ability to control fluid loss intoa formation.

Another important feature of fluids utilized in the subterraneandrilling industry is shear stability or stability under shear stress.The term "shear" is intended to refer to the ratio between a stress(force per unit area) applied laterally to a material and the strainresulting from this force. Determination of this ratio is one method ofmeasuring the viscosity of a liquid or semisolid. One manner ofmeasuring shear stability involves a determination of ζ₅₀. To determineζ₅₀ of an inventive composition, ξ₅₀ of a mixed salt composition isdetermined and related to the ξ₅₀ of one or more of the related singlesalt systems according to the following equation: ##EQU2## ξ₅₀represents the time, in an experiment conducted at no greater than T₅₀,at which sustained shear degradation at a shear stress rate of at least1021 sec⁻¹ causes the mixture viscosity to drop to 50% of its initialvalue at that same temperature (T₅₀). The viscosity is measured at bothinitial and final conditions at a shear stress rate of 170 sec⁻¹. In oneaspect of the invention, inventive two salt systems exhibit a ζ₅₀ ofgreater than about 1. This means that the ξ₅₀ of the inventive two saltsystem is greater than expected based upon the linear relationshipbetween the ξ₅₀ values for the two corresponding single salt systems. Asbefore, there are two corresponding single salt systems for eachinventive two salt system.

Another manner of measuring shear stability involves a determination ofω₅₀. To determine ω₅₀ of an inventive composition, σ₅₀ of a mixed saltcomposition is determined and related to the σ₅₀ of one or more of therelated single salt systems according to the following equation:##EQU3## σ₅₀ represents the shear rate, in an experiment conducted at atemperature no greater than T₅₀, at which sustained shear degradationfor at least 30 minutes causes the mixture viscosity to drop to 50% ofits initial value at that same temperature (T₅₀). The viscosity ismeasured at both initial and final conditions at a shear stress rate of170 sec⁻¹. In one aspect of the invention, inventive two salt systemsexhibit a ω₅₀ of greater than about 1. This means that the σ₅₀ of theinventive two salt system is greater than expected based upon the linearrelationship between the σ₅₀ values for the two corresponding singlesalt systems.

Another important feature of fluids utilized in the subterraneandrilling industry is fluid loss control. The term "fluid loss" isintended to refer to the leak-off of fluid into the rock matrix. Onemanner of measuring fluid loss control involves a determination of ψ₅₀.To determine ψ₅₀ of an inventive composition, ρ₅₀ of a mixed saltcomposition is determined and related to the ρ₅₀ of one or more of therelated single salt systems according to the following equation:##EQU4## ρ₅₀ represents the ratio of the API standard fluid loss volume,in an experiment conducted at a temperature no greater than T₅₀ after 16hours of rolling heat aging at a temperature no greater than T₅₀. In oneaspect of the invention, inventive two salt systems exhibit a ψ₅₀ ofless than about 1. This means that the ρ₅₀ of the inventive two saltsystem is less than expected based upon the linear relationship betweenthe ρ₅₀ values for the two corresponding single salt systems. As before,there are two corresponding single salt systems for each inventive twosalt system.

Another important feature of fluids utilized in the subterraneandrilling industry is pH stability. It is widely known to those skilledin the art that such processes as acid- or base- catalyzed hydrolyticdegradation can lead to loss of viscosifying capability on the part ofthe polymer. Accordingly, the process of formulating a viscosified fluidinvolves searching for a pH range wherein these chemical degradationprocesses are held to acceptable rates and then maintaining the fluid pHwithin that range throughout subsequent use of the viscosified fluid.The term "pH stability" is intended to refer to the resistance of thefluid to change in pH over time. One manner of measuring pH stabilityinvolves a determination of Φ. To determine Φ of an inventivecomposition, φ of a mixed salt composition is determined and related tothe φ of one or more of the related single salt systems according to thefollowing equation: ##EQU5## φ represents a ratio of viscosities and pHchanges determined as follows: The fluid is formulated and its initialproperties, pH and viscosity at 170 sec⁻¹, are measured. Then the fluidis subjected to 16 hours of rolling heat aging at a temperature nogreater than T₅₀ ; and the final properties, including pH and viscosityat 170 sec⁻¹, are measured. The parameter φ is then determined from thefollowing equation: ##EQU6## The φ values for the single salt andmultiple salt systems are measured similarly and used to measure Φaccording to the above equation. In one aspect of the invention,inventive two salt systems exhibit a Φ of greater than about 1. Thismeans that the φ₅₀ of the inventive two salt system is greater thanexpected based upon the linear relationship between the φ values for thetwo corresponding single salt systems.

In use, inventive compositions are preferably utilized in exploitingsubterranean petroliferous formations as drilling, drill-in, completion,hydraulic fracturing, work-over, packer, well treating, testing, spaceror hole abandonment fluids. For instance, in a preferred use as ahydraulic fracturing fluid, a viscosified mixed salt brine fluid isintroduced into a formation at a rate and pressure sufficient tofracture the formation. At first, the fluid leaks off into the rockmatrix, building up a filter cake on the rock face. The filter cake thenprevents fluid injected thereafter from leaking off significantly. Thefull force of the applied hydraulic pressure eventually comes to bearupon the rock face, causing the rock to part at the weakest point. Asthe fracture grows, additional fracturing fluid containing a solidproppant material is introduced. Following this treatment, as much aspossible of the introduced fluid is recovered from the formation, butthe proppant remains to prevent the complete closure of the fracture.The propped fracture creates a highly conductive channel extending fromthe well bore into the formation, making the reservoir more productive.

High viscosity fracturing fluids undergo high shear stress during theintroduction of such fluids into a formation. The viscosity of the fluidmust be high enough to carry proppant but low enough that excessivefriction losses and high well head pumping pressures are notencountered. Polymer degradation is a natural result of shear stressesimposed by pumping, the presence of abrasive materials, and high flowrates through small flow channels. This mechanical degradationaccompanies thermal degradation and chemical degradation produced byacid catalyzed hydrolysis of the acetal bonds which are the weakestlinks along the polymer backbone.

The invention will be further described with reference to the followingspecific Examples. It will be understood that these Examples areillustrative and not restrictive in nature.

EXAMPLE ONE PREPARING INVENTIVE DENSE, VISCOSIFIED AQUEOUS COMPOSITIONS

Two salts are selected and prepared by dissolving the salts into water.The following salt combinations are used:

lithium chloride/lithium bromide

calcium chloride/calcium bromide

sodium bromide/calcium bromide

lithium bromide/sodium bromide

lithium bromide/calcium bromide

sodium chloride/potassium chloride

sodium chloride/calcium bromide

sodium chloride/sodium bromide

sodium bromide/potassium bromide

sodium formate/sodium nitrate

sodium formate/potassium nitrate

potassium formate/sodium nitrate

sodium formate/cesium nitrate

cesium formate/sodium nitrate

potassium formate/cesium nitrate

cesium formate/potassium nitrate

lithium formate/cesium nitrate

cesium formate/lithium nitrate

potassium formate/calcium nitrate

calcium formate/potassium nitrate

potassium formate/potassium nitrate

cesium formate/cesium nitrate

potassium chloride/potassium bromide

potassium bromide/calcium bromide

potassium chloride/calcium bromide

potassium formate/calcium bromide

magnesium chloride/calcium chloride

magnesium chloride/calcium bromide

magnesium bromide/calcium bromide

magnesium chloride/magnesium bromide

cesium chloride/cesium bromide

cesium chloride/sodium chloride

cesium bromide/sodium chloride

cesium chloride/sodium bromide

cesium bromide/sodium bromide

cesium chloride/potassium chloride

cesium bromide/potassium chloride

cesium chloride/potassium bromide

cesium bromide/potassium bromide

cesium chloride/calcium chloride

cesium bromide/calcium chloride

cesium chloride/calcium bromide

cesium bromide/calcium bromide

calcium chloride/calcium formate

calcium bromide/calcium nitrate

calcium bromide/cesium bromide

zinc complex salts/alkali metal or alkaline earth metal salts

After the salts are dissolved into the water to a density of at leastabout 9.5, a water-soluble or water-dispersible polymer is hydrated intothe solution, thus viscosifying the solution for advantageous use as adrilling, drill-in, completion, hydraulic fracturing, work-over, packer,well treating, testing, spacer, or hole abandonment fluid. Amounts ofeach salt in relation to one another are determined such that theresulting viscosified fluid has a τ₅₀ of at least about 1; a ζ₅₀ of atleast about 1; a ω₅₀ of at least about 1; a ψ₅₀ of at most about 1; or aΦ of at least about 1. Preferably, the ratio of the first salt to thesecond salt in inventive two salt systems is from about 50/1 to about1/50.

EXAMPLE TWO PREPARING INVENTIVE DENSE, VISCOSIFIED AQUEOUS COMPOSITION

Inventive dense, viscosified aqueous compositions are prepared accordingto Example 1 except that the polymer is hydrated into a solution havingdissolved therein only one of the salts of a two salt combination. Afterhydration of the polymer, the second salt in the two salt composition isadded either in the form of a dry salt or in a salt solution.

EXAMPLE THREE TESTING PROPERTIES OF INVENTIVE DENSE, VISCOSIFIED AQUEOUSCOMPOSITIONS

Viscosity Measurements

A fluid is prepared as in Example 1 or Example 2 and Theologicalproperties are measured and recorded at 120° F. using a rotationalviscometer such as the Fann 35 at the following rpms: 600, 300, 200,100, 6 and 3. Also measured and recorded are the pH of the fluid at 120°F., and the 10 second and 10 minute gels.

Determining Heat Stability

One "barrel equivalent" (BEQ) of the fluid is hot rolled at apredetermined temperature for 16 hours; and one BEQ of the fluid is hotrolled at the predetermined temperature for 72 hours (or 48/96 hours iftime is available for longer testing). A normal 42 gallon barrel has aweight in pounds equal to (42×ρ), where ρ is the density of the fluid inpounds per gallon. The BEQ is obtained by simply replacing the weight ofthe barrel in pounds by the weight of the barrel equivalent in grams.This replacement scales the convenient field unit, the barrel, into aworkable laboratory size unit, BEQ. After the rheological properties arerecorded from the Fann 35 or equivalent for the 16/72 hour or 48/96 hoursamples, as indicated above, the samples are examined for evidence ofsag, break, or settling and the visual observations are logged. The gelsof hot rolled samples of inventive compositions are preferably withinthe range of from about 6 to about 19.

Fluid Loss Testing

The fluid loss properties of the fluid are determined as follows. Afluid loss test of duration ranging from 30 minutes to 24 hours isperformed in the API standard high pressure high temperature (HP-HT)apparatus. If additional time for testing is available, a 48 hour testduration may be preferable. The testing temperature is predetermined,such as, for example, in accordance with the bottom-hole temperature atwhich the fluid will be used in the field. The HP-HT apparatus isemployed using 250 to 600 psig differential pressure, using, forexample, a 50-2000 millidarcy Aloxite disc of diameter of 2.50 inchesand thickness of 0.25 inches.

The cell in the heating jacket and the fluid in the static oven arepreheated until the cell and sample reach the test temperature. Whilethe cell is being heated, the disc is covered with a thin layer ofde-ionized water. The cover (lid) for the cell should also be put inplace to prevent evaporation.

The fluid being tested is preheated and then poured into the preheatedcell. The loaded cell is then prepared for the application of pressure.The cell is pressurized and the temperature adjusted to the targettemperature. The discharge valve is then opened, and the filtrate volumeis recorded at time intervals such as 1, 5, 7.5, 10, 15, and 30 minutes,and at 1, 2, 4, 16, 24, and/or 48 hours. The fluid loss volume may thenbe plotted versus (time)^(1/2). This plot is, in most cases,substantially linear. If the plot is linear, the fluid loss behavior isconsidered normal. Whether or not this is the case is noted in thelaboratory notebook.

At 24 or 48 hours, the test is stopped and the cell is cooled anddepressurized. It is then noted whether any sag, settling or break hasoccurred in the fluid in the cell during the pressurized phase oftesting. The fluid is then decanted and the filter cake thickness ismeasured.

EXAMPLE FOUR CaCl₂ /CaBr₂

The present inventors have discovered that the stability of biopolymersin a calcium-based brine is dramatically improved when the formation ofthe base brine is optimized with respect to the calcium chloride andcalcium bromide content. At equivalent densities, brine fluidscontaining calcium chloride and calcium bromide exhibit more stablerheological and gel structure properties with biopolymers than do thecorresponding pure calcium chloride brines or pure calcium bromidebrines. When the pH of the viscosified fluid is buffered with magnesiumoxide in the traditional manner, the rheology of calciumchloride/calcium bromide brines is considerably more stable than theequivalent pure calcium chloride system. For brine densities up to 13.0ppg or more, the stability of calcium chloride/calcium bromide systemsis greater than for the corresponding pure calcium bromide systems.

Rheological and gel structure data for 9.0 pounds per gallon (ppg), 10.5ppg and 13.0 ppg calcium-based brines are reported in Table 2, Table 3and Table 4, respectively. For each brine, the concentration of calciumchloride and calcium bromide are reported as a percent of the totalbrine salt present (%TS), which is varied from 100% calcium chloride to100% calcium bromide for the 9.0 and 10.5 ppg brines and from 45%calcium chloride for the 13 ppg fluid. The enhanced thermal stability,rheology and gel structure properties for the calcium chloride/calciumbromide brines are clearly depicted when compared with the correspondingpure salt brines--a result which is totally unexpected based onconventional wisdom in the relevant field.

                  TABLE 2                                                         ______________________________________                                        9.0 ppg Viscosified Brine                                                     2 ppg Bio-Polymer & 2 ppb MgO                                                 Heat Aged 16 & 72 Hours @ 180° F.                                      CaCl.sub.2, % TS                                                                       100       60        31      0                                        CaBr.sub.2, % TS                                                                       0         40        69      100                                      ______________________________________                                        Aging Hours                                                                            16     72     16   72   16   72   16   72                            PV/YP    5/19   5/15   5/15 5/15 5/21 5/17 6/17 5/16                          Gels     7/9    4/4    3/3  4/4  10/13                                                                              8/11 9/13 6/7                           pH       8.8    8.9    8.7  8.8  9.0  8.5  9.0  8.6                           Settling No     No     No   No   No   No   No   No                            ______________________________________                                         % TS  Specified salt as a percent of total soluble salt content.         

                                      TABLE 3                                     __________________________________________________________________________    10.5 ppg Viscosified Brine                                                    2 ppg Bio-Polymer & 2 ppb MgO                                                 Heat Aged 16 & 72 Hours @ 180° F.                                      CaCl.sub.2, % TS                                                                     100   86    60    31 0                                                 CaBr.sub.2, % TS                                                                     0     14    40    69 100                                               __________________________________________________________________________    Aging Hours                                                                          16 72 16 72 16 72 16 72 16 72                                          PV/YP  9/7                                                                              7/2                                                                              9/18                                                                             8/12                                                                             9/18                                                                             8/16                                                                             7/21                                                                             7/18                                                                             7/16                                                                             6/12                                        Gels   0/0                                                                              0/0                                                                              3/3                                                                              1/1                                                                              5/7                                                                              5/5                                                                              6/8                                                                              6/7                                                                              4/4                                                                              2/2                                         pH     7.4                                                                              7.6                                                                              7.7                                                                              7.8                                                                              8.3                                                                              8.3                                                                              8.6                                                                              8.4                                                                              8.1                                                                              8.5                                         Settling                                                                             Yes                                                                              Yes                                                                              No Yes                                                                              No No No No No Yes                                         __________________________________________________________________________     % TS  Specified salt as a percent of total soluble salt content.         

                                      TABLE 4                                     __________________________________________________________________________    13.0 ppg Viscosified Brine                                                    2 ppg Bio-Polymer & 2 ppb MgO                                                 Heat Aged 16 & 72 Hours @ 180° F.                                      CaCl.sub.2, % TS                                                                    45     22    16     0                                                   CaBr.sub.2, % TS                                                                    55     78    84     100                                                 __________________________________________________________________________    Aging Hours                                                                         16 72  16 72 16  72 16 72                                               PV/YP 21/28                                                                            24/33                                                                             17/21                                                                            19/18                                                                            16/15                                                                             15/15                                                                            14/15                                                                            -                                                Gels  9/10                                                                             9/10                                                                              5/6                                                                              3/5                                                                              1/1 1/1                                                                              1/1                                                                              -                                                pH    6.7                                                                              6.8 7.1                                                                              7.1                                                                              7.5 7.1                                                                              7.2                                                                              -                                                Settling                                                                            No No  Yes                                                                              Yes                                                                              Yes Yes                                                                              Yes                                                                              -                                                __________________________________________________________________________     % TS  Specified salt as a percent of total soluble salt content.         

These surprising results were again verified when a Fann Model 50evaluation was performed on the 10.5 ppg CaCl₂, 10.5 ppg CaBr₂, and 10.5ppg mixed CaCl₂ /CaBr₂ solutions, this type of test being well known inthe art. Results shown in FIG. 2 for the two pure halide systems displaya continuous rapid decline in brine viscosity with increasingtemperature. Unexpectedly, the mixed halide system shows higherviscosity than either pure halide at a given temperature for a broadrange from room temperature to well over 220° F. The brine systems wereinitially prepared by adding the biopolymer to the brines and thenhydrating each system with the same shearing treatment using a Silversonmixer. When the biopolymer is not completely hydrated, its viscosityincreases with heating as the biopolymer fully hydrates. In theseexperiments the Fann 50 supplies the heat. In the single-salt CaCl₂ andsingle-salt CaBr₂ experiments the viscosity did not increase when thefluids were heated, indicating that the biopolymer had been fullyhydrated. However, with the mixed salt CaCl₂ /CaBr₂ system, theviscosity did increase early in the experiment. In order to establishwhether this observation was the result of biopolymer hydration, theexperiment was repeated; but this time the CaCl₂ /CaBr₂ mixture wasblended from samples of viscosified CaCl₂ which had already been provento be fully hydrated and of viscosified CaBr₂ which had already beenproven to be fully hydrated. This new mixture was undeniably fullyhydrated, and yet the results of the repeat experiment were almostexactly as before. This shows that the increase in viscosity seen withthe two-salt mixture is not a hydration effect but a true indication ofincreased stability in the two-salt mixture in comparison with thecorresponding single-salt systems of equal density.

As was mentioned above, T₅₀ is defined as the temperature at which afluid's viscosity reaches 50% of its original value measured at aboutroom temperature. The surprising relationship between T₅₀ and percentCaBr₂ in the 10.5 ppg system is clearly demonstrated in FIG. 3.

Application of this invention to optimization of calcium-based brinescan result in viscosified brines having unexpected and greatly improvedstability. This substantial increase in stability permits the practicaluse of these brines for things such as drilling brines, drill-in fluids,fluid loss control pills and hydraulic fracturing fluids.

EXAMPLE FIVE NaBr/CaBr₂

The present inventors have discovered that by taking a CaBr₂ solutionwhich can be viscosified with xanthan gum and weighting the solution toa higher density with dry NaBr, instead of CaBr₂ or CaCl₂, the timerequired to reach maximum viscosity is reduced substantially. Results ofadding 3 pounds per barrel (ppb) of xanthan gum to 14.2 ppg solutions,one a pure CaBr₂ solution and the other a mixed CaBr₂ /NaBr solution,are shown in Table 5. Surprisingly, the two salt mixture began toviscosity after only 10 minutes of shearing, while the CaBr₂ solutionrequired significantly more time and shear. The yield point (YP) for asolution is a good measure for the degree of polymer activation. The YPfor the mixed CaBr₂ /NaBr salt system reached 12% of its final value inthe first 10 minutes while the pure CaBr₂ system had none of itsultimate value. The two salt mixture fully viscosified within 22 hoursof stirring after shearing while the CaBr₂ solution did not do so evenafter 42 hours of stirring.

                                      TABLE 5                                     __________________________________________________________________________    14.2 ppg Solutions + 3 ppb of Xanthan Gum                                     CaBr.sub.2                                                                             CaBr.sub.2 /NaBr                                                                    CaBr.sub.2                                                                          CaBr.sub.2 /NaBr                                                                    CaBr.sub.2                                                                        CaBr.sub.2 /NaBr                                                                    CaBr.sub.2                                                                         CaBr.sub.2 /NaBr                    __________________________________________________________________________    600 22   44    30    71    83  144   104  148                                 300 11   26    16    47    56  106   72   108                                 6   0    2     0     6     7   24    13   25                                  3   0    1     0     4     5   19    9    20                                  Gels                                                                              0/0  1/1   0/0   4/5   5/8 20/48 10/21                                                                              24/56                               PV  11   18    14    24    27  38    32   40                                  YP  0    8     2     23    29  68    40   68                                  Method                                                                            Sheared-                                                                           10 Min.                                                                             Sheared-                                                                            18 min.                                                                             Stirred -                                                                         22 hours                                                                            Stirred -                                                                          42 hours                                Time       Time        Time      Time                                         RPM  4000  RPM   4000                                                     __________________________________________________________________________

FIG. 4 shows the T_(m) for 12.5 ppg NaBr, mixtures of 13.0 ppg CaBr₂/NaBr, and viscosity data for pure CaBr₂. The 13.0 ppg pure CaBr₂solution shows no T_(m), but with the introduction of NaBr, the T_(m)increases with increasing concentrations of NaBr. In FIG. 5, T₅₀ isplotted versus the pounds of NaBr per barrel (ppb) of fluid.Unexpectedly, NaBr was found to substantially increase the T₅₀ of themixed CaBr₂ /NaBr brine fluids. This shows that the mixed salt systemprovides higher thermal stability than the pure CaBr₂ solution.

To determine whether the mixed CaBr₂ /NaBr salt system also increasesthe thermal stability of other polysaccharides, such as pre-gelatinizedstarch, which show no transition temperature, a heating and coolingcycle was conducted on 12 ppb of a pre-gelatinized starch added to a13.0 ppg pure CaBr₂ solution and compared with 12 ppb of pre-gelatinizedstarch added to a 13.0 ppg mixed CaBr₂ /NaBr salt system. The mixed saltsystem was prepared by taking 11.0 ppg CaBr₂ and weighting it up withdry NaBr until a 13.0 ppg solution was obtained. Each solution washeated at 2.56° F./minute in 20° F. increments at a shear rate of 170sec⁻¹ beginning at 120° F. and then quick-cooled to 70° F. By using thisheating and cooling cycle, the breakdown of the pre-gelatinized starchwas determined and is shown in FIG. 6. The percent viscosities of bothsolutions begin to decrease to about 150° F. and then stop decreasing.However, when heated to 240° F. and cooled to 70° F., thepre-gelatinized starch in the 13.0 ppg pure CaBr₂ solution showeddramatic decrease in the retained percent viscosity while in the mixedsalt system this phenomena occurred only after the higher temperature of260° F. As such, FIG. 6 shows that the mixed salt system increases thethermal stability of the pre-gelatinized starch compared with the pureCaBr₂ solution.

EXAMPLE SIX LiBr/NaBr

Table 6 presents T₅₀ data for the LiBr/NaBr mixed salt system. Twodifferent ratios of LiBr to NaBr were tested and in both cases the datashow improved stability for the xanthan-viscosified LiBr/NaBr mixed saltsystem in comparison with viscosified pure LiBr and viscosified pureNaBr. For example, if one interpolates linearly between the T₅₀ for 12.5ppg LiBr, 114° F., and the T₅₀ for 12.5 ppg NaBr, 291° F. to predict theT₅₀ of the mixture, the actually observed T₅₀ values for the LiBr/NaBrmixtures are greater than those predicted. This comparison isillustrated graphically in FIG. 7 in which the T₅₀ values are plottedversus the pounds of LiBr added per barrel of fluid. The slightly upwardcurvature of the line joining pure NaBr (at 0) and pure LiBr shows theextra stabilization of the system when the brines are mixed.

                  TABLE 6                                                         ______________________________________                                        T.sub.50 Data on                                                              Mixed Salts                                                                   Mixed Salt    T.sub.50(A/B)                                                                              T.sub.50(A)                                                                          T.sub.50(B)                                 System A/B    (° F.)                                                                              (° F.)                                                                        (° F.)                               ______________________________________                                        LiBr/NaBr (50/50)                                                                           220          114    291                                         LiBr/NaBr (25/75)                                                                           263          114    291                                         ______________________________________                                    

EXAMPLE SEVEN LiBr/CaBr₂

Table 7 presents T₅₀ data for the LiBr/CaBr₂ mixed salt system. WhenLiBr was added to CaBr₂, extreme improvement was observed in thestability for the xanthan-viscosified LiBr/CaBr₂ mixed salt system incomparison with viscosified pure LiBr and viscosified pure CaBr₂. Forexample, as one can see clearly in FIG. 8, observed data issignificantly greater than that predicted by linear interpolation.

                  TABLE 7                                                         ______________________________________                                        T.sub.50 Data on                                                              Mixed Salts                                                                   Mixed Salt    T.sub.50(A/B)                                                                              T.sub.50(A)                                                                          T.sub.50(B)                                 System A/B    (° F.)                                                                              (° F.)                                                                        (° F.)                               ______________________________________                                        LiBr/CaCl.sub.2                                                                             266          114    184.8                                       ______________________________________                                    

EXAMPLE EIGHT NaCl/KCl

Table 8 presents T₅₀ data for the 9.0 ppg NaCl/KCl mixed salt system.The data showed no improvement in heat stability for thexanthan-viscosified NaCl/KCl mixed salt system in comparison withviscosified pure NaCl and viscosified pure KCl. The data obtained forthis two-salt system show no improvement over the corresponding singlesalt systems. Accordingly, in those systems wherein there is anobservable improvement with the mixed salt systems over thecorresponding single salts, said improvement is unexpected.Additionally, various systems which show no improvement in heatstability do show, for example, good shear stability and/or fluid losscontrol characteristics.

                  TABLE 8                                                         ______________________________________                                        T.sub.50 Data on                                                              Mixed Salts                                                                   Mixed Salt    T.sub.50(A/B)                                                                              T.sub.50(A)                                                                          T.sub.50(B)                                 System A/B    (° F.)                                                                              (° F.)                                                                        (° F.)                               ______________________________________                                        NaCl/KCl      300          300    300                                         ______________________________________                                    

EXAMPLE NINE NaCl/NaBr

As with the CaCl₂ /CaBr₂ system discussed above, the rheological andfluid loss data presented in Table 9 demonstrate the increasedeffectiveness and stability with time of the mixed halide salt systemwhen compared to the single NaCl salt system. This unexpected stabilityoccurs in an especially important temperature range and greatly extendsthe useful range for these drilling brine systems.

                                      TABLE 9                                     __________________________________________________________________________    9.5 ppg Sodium-based Drilling Brines                                                   NaCl      NaCl                                                                             NaCl/NaBr                                                                            NaCl                                                                             NaCl/NaBr                                              Test                                                                             NaCl/NaBr                                                                            Time                                                                             16 Hours                                                                             Time                                                                             72 Hours                                      System   Temp                                                                             Room   Temp                                                                             260° F.                                                                       Temp                                                                             260° F.                                __________________________________________________________________________    Rheological                                                                   Properties                                                                    6/3 RPM  25/23                                                                            23/22  23/21                                                                            22/20  9/7                                                                              17/15                                         Gels     24/27                                                                            22/25  23/20                                                                            20/24  7/8                                                                              15/16                                         PV       17 19     9  9      10 11                                            YP       49 47     45 43     24 37                                            Sample Temp, ° F.                                                               76 77     120                                                                              120    120                                                                              120                                           HTHP                                                                          Fluid Loss Test                                                               Temperature, ° F.                                                                         260                                                                              260                                                     Delta              500                                                                              500                                                     Pressure, psi      500                                                                              500                                                     Core, md                                                                      30 Minutes, ml     15 4                                                       48 Hours, ml       85 73                                                      __________________________________________________________________________

Table 10 presents T_(m) and T₅₀ data for the 9.0 ppg NaCl/NaBr mixedsalt system. The data for pure NaBr are estimated. The analysis suggestsno improvement in stability for the xanthan-viscosified NaCl/NaBr mixedsalt system in comparison with the corresponding viscosified pure singlesalts, in contrast with the Table 9 data just discussed above.

                  TABLE 10                                                        ______________________________________                                        T.sub.m and T.sub.50 Data on                                                  Mixed Salts                                                                   Mixed Salt                                                                            T.sub.m(A/B)                                                                           T.sub.m(A)                                                                            T.sub.m(B)                                                                         T.sub.50(A/B)                                                                        T.sub.50(A)                                                                         T.sub.50(B)                        System A/B                                                                            (° F.)                                                                          (° F.)                                                                         (° F.)                                                                      (° F.)                                                                        (° F.)                                                                       (° F.)                      ______________________________________                                        NaCl/NaBr                                                                             272      272     ≈270                                                                       300    300   ≈290                       ______________________________________                                    

What is the resolution to this apparent contrast? First, the Table 9data showed little or no difference at 16 hours, and only at 72 hoursdid a significant difference appear; the data in Table 10 are from testswhich span less than two hours. If differences only appear after anextended duration they cannot be seen in data such as those of Table 10.Second, the Table 10 T_(m) and T₅₀ data are all relatively hightemperatures and the densities of the brines involved are all relativelylow, 9.0 ppg. For other mixed brine systems, we have such data forgenerally higher brine densities, and often observe T_(m) and T₅₀ valuesfor the viscosified pure salts which are much lower than those of Table10. In mixed salt brines having higher densities and lower T_(m) and T₅₀values for the viscosified pure salts, it is apparently easier toobserve the benefit of improved stability due to the presence of themixed brines when observing over the course of short experiments such asthose of Table 10. The same phenomenon likely explains the Table 8 T₅₀data for the 9.0 ppg NaCl/KCl mixed salt system.

EXAMPLE TEN NaBr/KBr

Table 11 presents T₅₀ data for the 11.4 ppg NaBr/KBr mixed salt system.The data for pure NaBr are estimated. The analysis shows improvement instability for the xanthan-viscosified NaBr/KBr mixed salt system incomparison with the corresponding viscosified pure single salts.

                  TABLE 11                                                        ______________________________________                                        T.sub.50 Data on                                                              Mixed Salts                                                                   Mixed Salt   T.sub.50(A/B)                                                                             T.sub.50(B)                                                                           T.sub.50(B)                                  System A/B   (° F.)                                                                             (° F.)                                                                         (° F.)                                ______________________________________                                        NaBr/KBr (50/50)                                                                           300         <291    300                                          ______________________________________                                    

EXAMPLE ELEVEN NaHCO₂ /NaNO₃

Table 12 presents T_(m) data for the 11.0 ppg NaHCO₂ /NaNO₃ mixed saltsystem. The data showed substantial improvement in stability for thexanthan-viscosified NaHCO₂ /NaNO₃ mixed salt system in comparison withviscosified pure NaHCO₂ and viscosified pure NaNO₃. The 11.0 ppg mixtureof NaHCO₂ /NaNO₃ was a 50/50 mixture by weight of 11.0 ppg NaHCO₂ whichhas a T_(m) of 356° F. and 11.0 ppg NaNO₃ which has a T_(m) of 290° F.Interpolation between these data predicts a T_(m) of 323° F., whereas asubstantially higher value, 330° F., was observed. This comparison isillustrated graphically in FIG. 9 in which the T₅₀ values are plottedversus the pounds of NaNO₃ added per barrel of fluid. The slightlyupward curvature of the line joining pure NaHCO₂ (at 0) and pure NaNO₃shows the extra stabilization of the system when the brines are mixed.

                  TABLE 12                                                        ______________________________________                                        T.sub.50 Data on                                                              Mixed Salts                                                                   Mixed Salt   T.sub.M(A/B)                                                                              T.sub.M(A)                                                                            T.sub.M(B)                                   System A/B   (° F.)                                                                             (° F.)                                                                         (° F.)                                ______________________________________                                        NaHCO.sub.2 /NaNO.sub.3                                                                    330         356     290                                          ______________________________________                                    

EXAMPLE TWELVE KBr/CaBr₂, KHCO₂ /CaBr₂, and KBr/Ca(HCO₂)₂

Table 13 presents T₅₀ data for five mixed salt systems: a 13.0 ppgLiBr/CaBr₂ mixed salt system, a 13.0 ppg NaBr/CaBr₂ mixed salt systems,a 13.0 ppg KBr/CaBr₂ mixed salt system, a 13.2 ppg KHCO₂ /CaBr₂ mixedsalt system, and a 13.2 ppg KBr/Ca(HCO₂)₂ mixed salt system. TheLiBr/CaBr₂ and the NaBr/CaBr₂ mixed salt systems have been discussedabove. We were able to produce a solution comprising 42.3 weight % CaBr₂and 4.7 weight % KHCO₂ and having a density of 13.2 ppg. Adding just alittle more KHCO₂ would have led to formation of a precipitate; even so,it was found that the T₅₀ of the xanthan-viscosified KHCO₂ /CaBr₂ mixedbrine is substantially greater than the T₅₀ of a comparablexanthan-viscosified CaBr₂ brine.

The table also shows that the T₅₀ of the xanthan-viscosified KBr/CaBr₂mixed brine is substantially greater than the T₅₀ of a comparablexanthan-viscosified CaBr₂ brine.

                  TABLE 13                                                        ______________________________________                                        T.sub.50 Data on                                                              Mixed Salts                                                                          Mixed Salt        T.sub.50(A/B)                                                                         T.sub.50(A)                                                                         T.sub.50(B)                            Density                                                                              System A/B        (° F.)                                                                         (° F.)                                                                       (° F.)                          ______________________________________                                        13.0 ppg                                                                             LiBr/CaBr.sub.2   266     114   185                                    13.0 ppg                                                                             NaBr/CaBr.sub.2 (11.0 ppg + NaBr)                                                               273     --    185                                    13.0 ppg                                                                             KBr/CaBr.sub.2    261           185                                    13.2 ppg                                                                             KHCO.sub.2 /CaBr.sub.2                                                                          231           185                                    13.2 ppg                                                                             KBr/Ca(HCO.sub.2).sub.2                                                                         231                                                  ______________________________________                                    

EXAMPLE THIRTEEN MgCl₂ /CaCl₂

Calcium chloride at 10.5 ppg was converted into a drilling brine by theaddition of viscosifying polymers and conventional brine-based buffer at2 pounds per barrel. Similar MgCl₂ /CaCl₂ systems were prepared whereinsome of the calcium chloride was replaced with an equivalent amount ofmagnesium chloride. The resulting drilling brines were then hot rolledat 180° F. for 16 and 72 hours, and their drilling properties weremeasured. The resulting data are shown in Table 14. The trends observedfor the YP and Gel structure indicate the increased stability obtainedwith the mixed salt brine system compared to the pure CaCl₂ system. Thisunexpected increase in stability is amply demonstrated with the plot ofYP versus pounds MgCl₂ in the drilling brine, shown in FIG. 10.

                  TABLE 14                                                        ______________________________________                                        10.5 ppg CaCl.sub.2 /MgCl.sub.2 Drilling Brines                               Rheological and Gel Properties                                                Pounds MgCl2 per Barrel                                                                     0       4.0     14.1  18.7  28.1                                ______________________________________                                        16 Hours @ 180° F.                                                     6/3 RPM       1/1     6/4     7/5   6/5   8/6                                 Gels          0/0     5/5     5/6   5/5   7/8                                 PV            9       10      9     9     9                                   YP            7       19      20    19    21                                  Sample Temperature, ° F.                                                             121     120     120   120   120                                 72 Hours @ 180° F.                                                     6/3 RPM       0/0     1/0     2/1   3/2   3/2                                 Gels          0/0     0/0     2/2   2/3   3/3                                 PV            7       9       9     9     10                                  YP            2       9       13    15    15                                  Sample Temperature, ° F.                                                             122     120     120   120   120                                 ______________________________________                                    

EXAMPLE FOURTEEN MgCl₂ /CaBr₂ and MgBr₂ /CaCl₂

Brine based drilling fluids at 11.0 ppg were prepared from puremagnesium chloride and pure calcium bromide. Each fluid included thesame amount of biopolymer and conventional brine buffer (2 ppg), and wasprepared in an identical manner. Mixed brine fluids were prepared bymixing the appropriate quantities of each formulated pure salt system.The drilling brine fluids presented in Tables 15-17 were prepared asdescribed, subjected to the aging and heat aging conditions specified,and then the rheological and gel properties were measured and recorded.Quite unexpectedly the 35% MgCl₂ /65% CaBr₂ heat aged sample exhibitedsubstantially increased values for properties across the board comparedto either pure salt system. This fluid exhibited both increased gel andlow end rheological properties, and the properties of this system couldeasily be optimized even with reduced polymer content. Even the 65%MgCl₂ /35% CaBr₂ heat aged sample yielded improved properties comparedto the pure CaBr₂ system.

                  TABLE 15                                                        ______________________________________                                        11.0 ppg MgCl.sub.2 /CaBr.sub.2 -based Drilling Brines                        Initial Rheological Properties                                                           100%    65/35%     35/65%   100%                                   System     MgCl.sub.2                                                                            MgCl.sub.2 /CaBr.sub.2                                                                   MgCl.sub.2 /CaBr.sub.2                                                                 CaBr.sub.2                             ______________________________________                                        Rheological                                                                              Room    Room       Room     Room                                   Properties Temp    Temp       Temp     Temp                                   6/3 RPM    20/18   15/14      12/11    6/8                                    Gels       21/24   14/19      11/16    12/13                                  PV         31      25         16       11                                     YP         38      35         29       22                                     Sample Temp, ° F.                                                                 120     120        120      120                                    ______________________________________                                    

                  TABLE 16                                                        ______________________________________                                        11.0 ppg MgCl.sub.2 /CaBr.sub.2 -based Drilling Brines                        Heat-aged Rheological Properties                                                       100%     65/35%     35/65%   100%                                    System   MgCl.sub.2                                                                             MgCl.sub.2 /CaBr.sub.2                                                                   MgCl.sub.2 /CaBr.sub.2                                                                 CaBr.sub.2                              ______________________________________                                        Rheological                                                                            16 Hours 16 Hours   16 Hours 16 Hours                                Properties                                                                             180° F.                                                                         180° F                                                                            180° F                                                                          180° F                           6/3 RPM  17/14    15/12      39/36    6/4                                     Gels     18/21    14/17      39/44    5/6                                     PV       22       23         61       12                                      YP       32       37         43       20                                      Sample Temp,                                                                           120      120        120      120                                     ° F.                                                                   ______________________________________                                    

                  TABLE 17                                                        ______________________________________                                        11.0 ppg MgCl.sub.2 /CaBr.sub.2 -based Drilling Brines                        Heat-aged Rheological Properties                                                       100%     65/35%     35/65%   100%                                    System   MgCl.sub.2                                                                             MgCl.sub.2 /CaBr.sub.2                                                                   MgCl.sub.2 /CaBr.sub.2                                                                 CaBr.sub.2                              ______________________________________                                        Rheological                                                                            62 Hours 62 Hours   62 Hours 62 Hours                                Properties                                                                             180° F.                                                                         180° F.                                                                           180° F.                                                                         180° F.                          6/3 RPM  19/16    19/16      33/29    6/4                                     Gels     18/20    16/19      27/29    6/6                                     PV       20       28         60       11                                      YP       36       42         30       21                                      Sample Temp,                                                                           120      120        120      120                                     ° F.                                                                   ______________________________________                                    

EXAMPLE FIFTEEN CaBr₂ /Ca(NO₃)₂

Table 18 presents T_(m) data for the 13.0 ppg CaBr₂ /Ca(NO₃)₂ mixed saltsystem. The T_(m) for the viscosified CaBr₂ /Ca(NO₃)₂ mixture issurprisingly higher than that of viscosified pure Ca(NO₃)₂. Thestabilization observed in the mixed salt system is also apparent fromthe fact that the mixture has a T_(m) value whereas viscosified pureCaBr₂ does not even show a transition from which to measure a T_(m).

                  TABLE 18                                                        ______________________________________                                        T.sub.m Data on                                                               Mixed Salts                                                                   Mixed Salt     T.sub.m(A/B)                                                                              T.sub.m(A)                                                                           T.sub.m(B)                                  System A/B     (° F.)                                                                             (° F.)                                                                        (° F.)                               ______________________________________                                        CaBr.sub.2 /Ca(NO.sub.3).sub.2 (50/50)                                                       225         N/A    222                                         ______________________________________                                    

EXAMPLE SIXTEEN Zinc Complex Salts Mixed with Alkali Metal and AlkalineEarth Metal Salts

Table 19 presents data for three mixed salt systems: a 17.5 ppg CaZnBr₄/Ca₃ (ZnBr₅)₂ /Ca₂ ZnBr₆ /CaBr₂ /NaBr mixed salt system (or moreconventionally represented as a CaBr₂ /ZnBr₂ /NaBr mixed salt system),and two different 17.5 ppg CaZnBr₄ /Ca₃ (ZnBr₅)₂ /Ca₂ ZnBr₆ CaBr₂ mixedsalt systems (or more conventionally represented as a CaBr₂ /ZnBr₂ mixedsalt system). These solutions were formulated initially from 19.2 ppgCaBr₂ /ZnBr₂ mixed salt clear brine and 13.2 ppg CaBr₂ ; then additionalsolid salt containing bromide anion was added to the formulation toassure that the zinc would be complexed to a significant extentaccording to the following chemical reaction:

    Zn.sup.2+ +U.sup.- +V.sup.- +W.sup.- +X.sup.- +Y.sup.- +Z.sup.- ⃡ZnUVWXYZ.sup.4-                              (1)

    Zn.sup.2+ +NH.sub.3 +NH.sub.3 +NH.sub.3 +NH.sub.3 +Y.sup.- +Z.sup.- ⃡Zn(NH.sub.3).sub.4 YZ°

    Zn.sup.2+ +NH.sub.3 +NH.sub.3 +NH.sub.3 +NH.sub.3 +NH.sub.3 +Z.sup.- ⃡Zn(NH.sub.3).sub.5 YZ.sup.+

where U⁻, V⁻, W⁻, X⁻, Y⁻, and Z⁻ are zinc complexing agents such as Cl⁻,Br⁻, I⁻, OH⁻, HCO₂ ⁻, CH₃ CO₂ ⁻, C₂ O₄ ²⁻, HCO₃ ⁻, CO₃ ²⁻, SCN⁻, CNO⁻,citrate anion, etc. Robert M. Smith, Critical Stability Constants,Volume 4, p. 115, Volume 5, p. 421, and Volume 6, page 459, (New York:Plenum Press, 1989). The above list can obviously be augmented withnon-ionic zinc complexing agents such as NH₃, H₂ S, CH₃ I, and variousother organics, such as, for example, primary amines likeisopropanolamine, secondary amines such as methyl ethanolamine, andtertiary amines, including triethanolamine. Additionally, two or more ofthe complexing agents, Y⁻ and Z⁻, may be supplied by different parts ofa single moiety. For example, both the citrate and oxalate anionmolecules comprise more than one carboxylate group, two of which can beattached as ligands to a given zinc ion just as easily as can eachcarboxylate group serve as a ligand for separate zinc ions.

Possible variants of Reaction 1 include the following:

    Zn.sup.2+ +V.sup.- +W.sup.- +X.sup.- +Y.sup.- +Z.sup.- ⃡ZnVWXYZ.sup.3-                               (1a)

and

    Zn.sup.2+ +W.sup.- +X.sup.- +Y.sup.- +Z.sup.- ⃡ZnWXYZ.sup.2-(1b)

In the Table 19 systems, U⁻, V⁻, W⁻, X⁻, Y⁻, and Z⁻ are all embodied inReaction 1 as the bromide anion and therefore ZnUVWXYZ⁴⁻ as the ZnBr₆ ⁴⁻anion, ZnVWXYZ³⁻ as the ZnBr₅ ³⁻ anion, ZnWXYZ²⁻ as the ZnBr₄ ²⁻ anion,etc. The reason for the need to get the bromide anion concentration highis seen in another variant of Reaction 1, given below as Reaction 2, andits further variants:

    Zn.sup.2+ +H.sub.2 O+V.sup.- +W.sup.- +X.sup.- +Y.sup.- +Z.sup.- ⃡Zn(OH)VWXYZ.sup.4- +H.sup.+                  (2)

    Zn.sup.2+ +H.sub.2 O+W.sup.- +X.sup.- +Y.sup.- +Z.sup.- ⃡Zn(OH)WXYZ.sup.3- +H.sup.+                   (2a)

    and Zn.sup.2+ +H.sub.2 O+X.sup.- +Y.sup.- +Z.sup.- ⃡Zn(OH)XYZ.sup.2- +H.sup.+                    (2b)

Since the Table 19 systems are water-based, there is therefore atendency for the zinc to hydrolyze in accordance with Reaction 2,releasing H⁺ and reducing the pH. At low pH, the thermal decompositionof biopolymers and common fluid loss control materials is accelerated.To avoid this undesirable consequence, consider the combination ofReactions 1 and 2, which is Reaction 3 and its variants, below:

    Zn(OH)VWXYZ.sup.4- +H.sup.+ +U.sup.- ⃡ZnUVWXYZ.sup.4- +H.sub.2 O(3)

    Zn(OH)WXYZ.sup.3- +H.sup.+ +V.sup.- ⃡ZnVWXYZ.sup.3- +H.sub.2 O(3a)

    and Zn(OH)XYZ.sup.2- +H.sup.+ +W.sup.- ⃡ZnWXYZ.sup.2- +H.sub.2 O(3b)

In water-based zinc brines, accordingly, having an excess of bromideanion or any other strongly zinc-complexing agent will, through theaction of LeChatelier's principle upon Reaction 3, shift the equilibriumto the right and diminish the concentration of H⁺. Notice in Table 19that the highest pH values are seen in the last column, where thehighest level of added salt and lowest rate of thermal decomposition ofbiopolymers and common fluid loss control materials are also to befound. Another undesirable consequence to avoid can be seen in furthervariants of Reaction 2, below:

    Zn(OH)VWXYZ.sup.4- +H.sub.2 O⃡Zn(OH).sub.2 WXYZ.sup.4- +H.sup.+ +V.sup.-                                                  (2c)

    Zn(OH)WXYZ.sup.3- +H.sub.2 O⃡Zn(OH).sub.2 XYZ.sup.3- +H.sup.+ +W.sup.-                                                  (2d)

    and Zn(OH)XYZ.sup.2- +H.sub.2 O⃡Zn(OH).sub.2 YZ.sup.2- +H.sup.+ +X.sup.-                                                  (2e)

Reactions 2c through 2e, and the like, are undesirable because theywould further reduce the pH. However, having an excess of bromide anionor any other strongly zinc-complexing agent to serve in the roles of V⁻,W⁻, and X⁻, in these reactions will, through the action, again, ofLeChatelier's principle shift the equilibria to the left and diminishthe concentration of H⁺.

Considering the center two columns in Table 19, it is apparent thatadding about 20 ppb of calcium bromide is somewhat more effective inincreasing the pH and extending the durability of the formulation thanadding about the same weight of NaBr.

It should be noted that while the added soluble salts were acting in therole of a pH buffer, conventional pH buffering agent was added to eachdrill-in fluid given in Table 19. The amount added would have been, formost other base brine systems, sufficient to keep the pH well above 5.Instead, none of the fluids in Table 19 had such a relatively high pH.This fact is a reflection of the strong pH control resulting from thebase brine chemistry (Reactions 1-3 and variants) and negligible pHcontrol induced by the presence of the 10 ppb of pH buffering agent. Thepoint is underscored that obtaining a desirably low rate of thermaldecomposition is possible especially by proper control of the pH throughcontrol of brine chemistry (especially Reactions 3 and variants), ratherthan through the addition of conventional pH buffering agents.

                                      TABLE 19                                    __________________________________________________________________________    17.5 ppg Viscosified ZnBr.sub.2 -based Brine                                  2 ppb Bio-Polymer, 4 ppb Fluid Loss Control Material, & 10 ppb pH Buffer      Heat Aged 16, 40, & 96 Hours @ 180° F.                                 __________________________________________________________________________    NaBr added, ppb                                                                        20.25       0           0                                            CaBr.sub.2 added, ppb                                                                  0           19.5        83.9                                         Base Brine System                                                                      17.5 ppg CaZnBr.sub.4 /                                                                   17.5 ppg CaZnBr.sub.4 /                                                                   17.5 ppg CaZnBr.sub.4 /                               Ca.sub.3 (ZnBr.sub.5).sub.2 /                                                             Ca.sub.3 (ZnBr.sub.5).sub.2 /                                                             Ca.sub.3 (ZnBr.sub.5).sub.2 /                         Ca.sub.2 ZnBr.sub.6 /NaBr                                                                 Ca.sub.2 ZnBr.sub.6 /CaBr.sub.2                                                           Ca.sub.2 ZnBr.sub.6 /CaBr.sub.2              Heat Aging Hours                                                                       16   40 96  16   40 96  16  40  96                                   PV/YP    53/75                                                                              49/34                                                                            --/--.sup.a                                                                       60/82                                                                              60/78                                                                            34/9.sup.b                                                                        104/80                                                                            118/84                                                                            15/82.sup.c                          Gels     23/17                                                                              7/5                                                                              --/--.sup.a                                                                       23/17                                                                              7/5                                                                              1/1.sup.b                                                                         19/19                                                                             16/17                                                                             11/12.sup.c                          pH       2.76 2.57                                                                             --.sup.a                                                                          3.10 2.82                                                                             2.74.sup.b                                                                        3.77                                                                              3.83                                                                              3.75.sup.c                           Settling No   No Yes No   No Yes No  No  No                                   Reference                                                                              BW3-62      BW3-58      RH2-6                                        __________________________________________________________________________     Note .sup.a : The pH and rheology were not measured because the biopolyme     and fluid loss control material bad very severely degraded at this point      in the rolling heataging.                                                     Note .sup.b : The biopolymer and fluid loss control material had degraded     but not so severely as in the case described in Note a.                       Note .sup.c : The biopolymer and fluid loss control material had degraded     but not so severely as in the case described in Note b.                  

Table 20 presents data for the following eleven mixed salt clear brinesystems:

A 15 ppg ZnBr₂ /CaBr₂ mixed salt system;

A 15 ppg CaZnBr₄ /Ca₂ ZnBr₆ /CaBr₂ mixed salt system;

A 15 ppg CaZnBr₄ /Ca₂ ZnBr₆ /CaBr₂ /NaBr mixed salt system;

A 15 ppg CaZnBr₄ /Ca₂ ZnBr₆ /CaBr₂ /CaCl₂ mixed salt system;

A 15 ppg CaZnBr₄ /Ca₂ ZnBr₆ /CaBr₂ /NaCl mixed salt system;

A 17 ppg ZnBr₂ /CaBr₂ mixed salt system;

A 17 ppg CaZnBr₄ /Ca₂ ZnBr₆ /CaBr₂ mixed salt system;

A 17 ppg CaZnBr₄ /Ca₂ ZnBr₆ /CaBr₂ /NaBr mixed salt system;

A 17 ppg CaZnBr₄ /Ca₂ ZnBr₆ /CaBr₂ /CaCl₂ mixed salt system;

A 17 ppg CaZnBr₄ /Ca₂ ZnBr₆ /CaBr₂ NaCl mixed salt system; and

A 17 ppg CaZnBr₄ /Ca₂ ZnBr₆ /CaBr₂ /CaI₂ mixed salt system.

                                      TABLE 20                                    __________________________________________________________________________    15.0 ppg and 17.0 ppg ZnBr.sub.2 /CaBr.sub.2 - and CaZnBr.sub.4 /Ca.sub.2     ZnBr.sub.6 -based Clear Brines                                                The Effect of Added Soluble Salt on pH                                                       Base Brine                                                                    Density                                                                              Weight-up                                                                           Weight Added                                      System         (ppg)  Chemical                                                                            (ppb)   pH                                        __________________________________________________________________________    ZnBr.sub.2 /CaBr.sub.2                                                                       15.0   None  0.0     3.07                                      CaZnBr.sub.4 /Ca.sub.2 ZnBr.sub.6 /CaBr.sub.2                                                15.0   CaBr.sub.2                                                                          329.9   5.37                                      CaZnBr.sub.4 /Ca.sub.2 ZnBr.sub.6 /CaBr.sub.2 /NaBr                                          15.0   NaBr  206.6   4.88                                      CaZnBr.sub.4 /Ca.sub.2 ZnB.sub.6 /CaBr.sub.2 /CaCl.sub.2                                     15.0   CaCl.sub.2                                                                          167.0   4.88                                      CaZnBr.sub.4 /Ca.sub.2 ZnBr.sub.6 /CaBr.sub.2 /NaCl                                          15.0   NaCl  109.3   4.30                                      ZnBr.sub.2 /CaBr.sub.2                                                                       17.0   None  0.0     2.09                                      CaZnBr.sub.4 /Ca.sub.2 ZnBr.sub.6 /CaBr.sub.2                                                17.0   CaBr.sub.2                                                                          206.1   4.19                                      CaZnBr.sub.4 /Ca.sub.2 ZnBr.sub.6 /CaBr.sub.2 /NaBr                                          17.0   NaBr  146.8   3.39                                      CaZnBr.sub.4 /Ca.sub.2 ZnBr.sub.6 CaBr.sub.2 /CaCl.sub.2                                     17.0   CaCl.sub.2                                                                          123.4   3.97                                      CaZnBr.sub.4 /Ca.sub.2 ZnBr.sub.6 /CaBr.sub.2 /NaCl                                          17.0   NaCl  90.8    3.19                                      CaZnBr.sub.4 /Ca.sub.2 ZnBr.sub.6 /CaBr.sub.2/CaI.sub.2                                      18.2   CaI.sub.2                                                                           330.0   3.71                                      __________________________________________________________________________

Comparisons of the various salt additives are best made on a molarbasis, as is seen below. In Table 21, we have expanded the Table 20 datafrom 11 systems to 48, by adding data from 37 systems very closelyrelated to the 11 from Table 20. Additionally, molar-basis data aregiven.

                                      TABLE 21                                    __________________________________________________________________________    15.0 ppg and 17.0 ppg ZnBr.sub.2 /CaBr.sub.2 - and CaZnBr.sub.4 /Ca.sub.2     ZnBr.sub.6 -based Clear                                                       Brines                                                                        The Effect of Added Soluble Salt on pH                                        Moles  Zinc                                                                        Moles  Ca.sub.2+                                                                  Moles  Na.sub.+                                                                    Moles  Cl.sub.-                                                                   Moles  Br.sub.-                                                                    Moles  I.sub.-                                                                     ##STR2## pH                                       __________________________________________________________________________    1.12 2.30                                                                              0    0   3.42 0   3.04      3.07                                     0.68 1.40                                                                              1.43 0   3.51 0   5.12      4.42                                     0.68 1.40                                                                              1.91 0   3.99 0   5.83      4.85                                     0.68 1.40                                                                              2.01 0   4.09 0   5.98      4.88                                     0.29 3.20                                                                              0    0   3.49 0   11.90     5.07                                     0.29 3.70                                                                              0    0   3.99 0   13.61     5.27                                     0.29 3.80                                                                              0    0   4.09 0   13.95     5.33                                     0.29 3.85                                                                              0    0   4.14 0   14.12     5.35                                     0.29 3.90                                                                              0    0   4.19 0   14.29     5.37                                     0.82 3.51                                                                              0    1.84                                                                              2.50 0   5.28      4.47                                     0.82 4.41                                                                              0    2.74                                                                              2.50 0   6.38      4.80                                     0.82 4.59                                                                              0    2.92                                                                              2.50 0   6.60      4.82                                     0.82 4.68                                                                              0    3.01                                                                              2.50 0   6.71      4.88                                     1.00 2.03                                                                              0.84 0.84                                                                              3.03 0   3.89      3.53                                     1.00 2.03                                                                              1.70 1.70                                                                              3.03 0   4.74      4.29                                     1.00 2.03                                                                              1.87 1.87                                                                              3.03 0   4.92      4.30                                     1.46 2.99                                                                              0    0   4.45 0   3.04      2.09                                     1.23 2.51                                                                              0.75 0   4.49 0   3.65      2.81                                     1.23 2.51                                                                              1.23 0   4.98 0   4.04      3.30                                     l.23 2.51                                                                              1.33 0   5.08 0   4.12      3.34                                     1.23 2.51                                                                              1.38 0   5.12 0   4.16      3.39                                     1.23 2.51                                                                              1.43 0   5.17 0   4.20      3.39                                     1.03 3.46                                                                              0    0   4.49 0   4.37      3.28                                     1.03 3.96                                                                              0    0   4.99 0   4.85      3.90                                     1.03 4.06                                                                              0    0   5.09 0   4.95      4.05                                     1.03 4.11                                                                              0    0   5.14 0   5.00      4.09                                     1.03 4.i6                                                                              0    0   5.19 0   5.05      4.i9                                     1.30 3.62                                                                              0    0.96                                                                              3.96 0   3.78      2.48                                     1.30 4.52                                                                              0    1.86                                                                              3.96 0   4.47      3.56                                     l.30 4.70                                                                              0    2.04                                                                              3.96 0   4.61      3.84                                     1.30 4.79                                                                              0    2.13                                                                              3.96 0   4.58      3.97                                     1.40 2.85                                                                              0.44 0.44                                                                              4.24 0   3.36      2.37                                     1.40 2.85                                                                              1.30 1.30                                                                              4.24 0   3.97      3.01                                     1.40 2.85                                                                              1.47 1.47                                                                              4.24 0   4.09      3.17                                     1.40 2.85                                                                              1.55 1.55                                                                              4.24 0   4.15      3.19                                     1.46 2.99                                                                              0    0   4.45 0   3.04      2.09                                     1.46 3.04                                                                              0    0   4.45 0.06                                                                              3.08      2.23                                     1.46 3.10                                                                              0    0   4.45 0.11                                                                              3.12      2.31                                     1.46 3.27                                                                              0    0   4.45 0.29                                                                              3.24      2.35                                     1.46 3.45                                                                              0    0   4.45 0.46                                                                              3.36      2.53                                     1.46 3.62                                                                              0    0   4.45 0.63                                                                              3.47      2.62                                     1.46 3.82                                                                              0    0   4.45 0.83                                                                              3.61      2.81                                     1.46 4.14                                                                              0    0   4.45 1.15                                                                              3.83      3.16                                     1.46 4.42                                                                              0    0   4.45 1.44                                                                              4.02      3.35                                     1.46 4.54                                                                              0    0   4.45 1.55                                                                              4.10      3.46                                     1.46 4.65                                                                              0    0   4.45 1.67                                                                              4.18      3.56                                     1.46 4.77                                                                              0    0   4.45 1.78                                                                              4.26      3.62                                     1.46 4.88                                                                              0    0   4.45 1.90                                                                              4.34      3.71                                     __________________________________________________________________________

The Table 21 data generally show a Ca/Zn molar ratio of 2 or greater,and as the molar ratio of total halides to zinc increases, the pH of thesolution increases, as is desired to decrease the thermal decompositionof the biopolymers and common fluid loss control materials normallyadded to brines used in drilling. FIG. 11 presents the data from thelast two columns in Table 21: pH is plotted versus the logarithm of themolar ratio of total halides to zinc. The logical justification of theform of the plot in FIG. 11 can be seen from considering Equation 4,which defines the function pZn-pX: ##EQU7## Since pH is -log[H⁺ ], afunction of similar form to pZn-pX, it makes sense to plot pH as thevertical axis in FIG. 11. The horizontal axis in FIG. 11, log {(TotalMoles X⁻)/(Moles Zn)}, is actually pZn-pX. Surprisingly, over threequarters of the data cluster about the straight line shown in FIG. 11 inthe vicinity ranging from pH 2 to about pH 4.5. Then the pH breaks overand appears to level off around pH 5.1 to 5.5. The transition occursaround log{X⁻ /Zn²⁺ }=pZn-pX=log{6}, i. e., around X⁻ /Zn²⁺ =6. Thisratio is consistent with an interpretation that a stable zinc complexanion stoichiometry, ZnX₆ ⁴⁻, has been reached in this vicinity; and lowpH forms like Zn(OH)X₅ ⁴⁻ have been minimized or even largelyeliminated, as indicated in Reaction 5, and its variants, below:

    Zn(OH)Br.sub.5.sup.4- +H.sup.+ +Br.sup.- ⃡ZnBr.sub.6.sup.4- +H.sub.2 O                                                (5)

    Zn(OH)Br.sub.5.sup.4- +H.sup.+ +Cl.sup.- ⃡ZnClBr.sub.5.sup.4- +H.sub.2 O                                                (5a)

    Zn(OH)Br.sub.5.sup.4- +H.sup.+ +I.sup.- ⃡ZnBr.sub.5 I.sup.4- +H.sub.2 O                                                (5b)

. .

    Zn(OH)ClBr.sub.4.sup.4- +H.sup.+ +Cl.sup.- ⃡ZnCl.sub.2 Br.sub.4.sup.4- +H.sub.2 O                                (5c)

    Zn(OH)Br.sub.4 I.sup.4- +H.sup.+ +I.sup.- ⃡ZnBr.sub.4 I.sub.2.sup.4- +H.sub.2 O                                 (5d)

. . .

    Zn(OH)Br.sub.4.sup.3- +H.sup.+ +Br.sup.- ⃡ZnBr.sub.5.sup.3- +H.sub.2 O                                                (5e)

    Zn(OH)Br.sub.4.sup.3- +H.sup.+ +Cl.sup.- ⃡ZnClBr.sub.4.sup.3- +H.sub.2 O                                                (5f)

    Zn(OH)Br.sub.4.sup.3- +H.sup.+ +I.sup.- ⃡ZnBr.sub.4 I.sup.3- +H.sub.2 O                                                (5g)

. .

    Zn(OH)ClBr.sub.3.sup.3- +H.sup.+ +Cl.sup.- ⃡ZnCl.sub.2 Br.sub.3.sup.3- +H.sub.2 O                                (5h)

    Zn(OH)Br.sub.3.sup.3- +H.sup.+ +I.sup.- ⃡ZnBr.sub.3 I.sub.2.sup.3- +H.sub.2 O                                 (5i)

. . .

    Zn(OH)Br.sub.3.sup.2- +H.sup.+ +Br.sup.- ⃡ZnBr.sub.4.sup.2- +H.sub.2                                                  (5j)

    Zn(OH)Br.sub.3.sup.2- +H.sup.+ +Cl.sup.- ⃡ZnClBr.sub.3.sup.2- +H.sub.2 O                                                (5k)

    Zn(OH)Br.sub.3.sup.2- +H.sup.+ +I.sup.- ⃡ZnBr.sub.3 I.sup.2- +H.sub.2 O                                                (5l)

. .

    Zn(OH)ClBr.sub.2.sup.2- +H.sup.+ +Cl.sup.- ⃡ZnCl.sub.2 Br.sub.2.sup.2- +H.sub.2 O                                (5m)

    and Zn(OH)Br.sub.2 I.sup.2- +H.sup.+ +I.sup.- ⃡ZnBr.sub.2 I.sub.2.sup.2- +H.sub.2 O                                 (5n)

. . .

EXAMPLE SEVENTEEN USES OF INVENTIVE DENSE, VISCOSIFIED AQUEOUSCOMPOSITIONS

Inventive compositions find advantageous use as replacements forconventional drilling, drill-in, completion, hydraulic fracturing,work-over, packer, well treating, testing, spacer, or hole abandonmentfluids, these uses being well known in the art to which the inventionpertains.

What is claimed is:
 1. A composition comprising:water; a water-solubleor water-dispersible polymer capable of viscosifying an aqueous medium;one or more cations including a member selected from the groupconsisting of lithium, sodium, potassium, cesium, magnesium, calcium,zinc, a zinc complex cation and mixtures thereof; and a zinc complexanion; wherein there are present either at least two cations or at leasttwo anions; the composition having a property selected from the groupconsisting of a τ₅₀ of at least about 1; a ψ₅₀ of at most about 1; a Φof at least about 1; a ζ₅₀ of at least about 1; and a ω₅₀ of at leastabout
 1. 2. The composition of claim 1, wherein said polymer is abiopolymer.
 3. The composition of claim 1, wherein said polymer isselected from the group consisting of algin; anionic cellulose; anionicpolysaccharide; cationic polysaccharide; carboxymethyl cellulose;carboxymethyl hydroxyethyl cellulose; gellan gum; guar gum; gum ghatti;gum karaya; gum tragacanth; gum arabic; gum acacia; locust bean gum;methacrylic acid polymer; polyamine; polyanionic cellulose; iota, lambdaor kappa sulfated polysaccharides; polysaccharides modified by i)cross-linking, ii) hydroxyethylation, iii) hydroxypropylation, iv)carboxymethyl-hydroxyethylation, v) carboxymethyl-hydroxypropylation,vi) carboxymethylation, or vii) carboxylation; rhamsan gum; vinylcompound polymer; wellan gum; glycol-compatible wellan gum; xanthan;xanthan gum; and mixtures of said polymers.
 4. The composition of claim1, wherein the composition comprises from about 0.01 to about 45.0percent polymer by weight; from about 10.0 to about 90.0 percent waterby weight; from about 0.05 to about 85.0 weight percent of a first salt;and from about 0.05 to about 85.0 weight percent of a second salt. 5.The composition of claim 1, wherein the composition comprises from about0.5 to about 10 percent polymer by weight; from about 10.0 to about 90.0percent water by weight; from about 2 to about 80 weight percent of afirst salt; and from about 2 to about 40 weight percent of a secondsalt.
 6. The composition of claim 1, wherein at least one of saidcations is held as a complex having cationic, neutral, or anionic form.7. The composition of claim 1, further comprising a chloride anion and abromide anion.
 8. The composition of claim 1, comprising a first cationhaving a +2 charge; a second cation having a +2 charge; and a thirdcation having a +1 charge.
 9. The composition of claim 1, comprising afirst cation having a +2 charge; a second cation which is held as acomplex having cationic, neutral, or anionic form; and a third cationhaving a +1 charge.
 10. The composition of claim 1, comprising calciumcations, and further comprising chloride anions and bromide anions. 11.The composition of claim 10, wherein the chloride to bromide ratio isfrom about 20/80 to about 80/20.
 12. The composition of claim 10,wherein the chloride to bromide ratio is from about 30/70 to about60/40.
 13. The composition of claim 1, comprising calcium cations andzinc cations.
 14. The composition of claim 1, comprising sodium cations,calcium cations and zinc cations.
 15. The composition of claim 1,wherein the zinc complex anion comprises a zinc cation having complexedtherewith a plurality of complexing agents selected from the groupconsisting of chloride, bromide, iodide, formate, acetate, citrate,oxalate, thiocyanate, cyanate, hydroxide, carbonate, bicarbonate,ammonia and amines.
 16. The composition of claim 15, wherein the zinccomplexing agent is bromide.
 17. The composition of claim 1, wherein thecomposition comprises a cation selected from the group consisting of acalcium cation, a zinc cation, a potassium cation and a sodium cation.18. The composition of claim 1, wherein said polymer is selected fromthe group consisting of xanthan and xanthan gum.
 19. A method forproviding a stably viscosified composition comprising:providing anaqueous solution comprising water having dissolved therein a first saltand a second salt, wherein the solution has a density of at least about9.5 pounds per gallon; mixing a water-soluble or water-dispersiblepolymer into the solution to yield a viscosified composition; andmaintaining a ratio of first salt to second salt within a range wherebythere is maintained in the viscosified composition a property selectedfrom the group consisting of a τ₅₀ of at least about 1; a ψ₅₀ of at mostabout 1; a Φ of at least about 1; a ζ₅₀ of at least about 1; and a ω₅₀of at least about
 1. 20. The method of claim 19, wherein the solutionhas a density of at least about 10 pounds per gallon.
 21. The method ofclaim 19, wherein the solution has a density of at least about 11.5pounds per gallon.
 22. The method according to claim 19, wherein thepolymer is a biopolymer.
 23. The method according to claim 19, whereinthe polymer is selected from the group consisting of xanthan and xanthangum.
 24. The method according to claim 19, wherein the polymer isselected from the group consisting of algin; anionic cellulose; anionicpolysaccharide; cationic polysaccharide; carboxymethyl cellulose;carboxymethyl hydroxyethyl cellulose; gellan gum; guar gum; gum ghatti;gum karaya; gum tragacanth; gum arabic; gum acacia; locust bean gum;methacrylic acid polymer; polyamine; polyanionic cellulose; iota, lambdaor kappa sulfated polysaccharides; polysaccharides modified by i)cross-linking, ii) hydroxyethylation, iii) hydroxypropylation, iv)carboxymethyl-hydroxyethylation, v) carboxymethyl-hydroxypropylation,vi) carboxymethylation, or vii) carboxylation; rhamsan gum; vinylcompound polymer; wellan gum; glycol-compatible wellan gum; xanthan;xanthan gum; and mixtures thereof.
 25. The method according to claim 19,wherein each of the first and second salts comprises a cation selectedfrom the group consisting of lithium, sodium, potassium, cesium,magnesium, calcium, zinc and mixtures thereof; and an anion selectedfrom the group consisting of chloride, bromide, iodide, formate,nitrate, acetate, cyanate, thiocyanate, a zinc complex anion andmixtures thereof.
 26. The method according to claim 25, wherein at leastone of the cations is held as a complex having cationic, neutral oranionic form.
 27. The method according to claim 25, comprising chlorideanions and bromide anions.
 28. The method according to claim 27, furthercomprising calcium cations.
 29. The method according to claim 27,further comprising sodium cations.
 30. The method according to claim 27,wherein the chloride to bromide ratio is from about 20:80 to about80:20.
 31. The method according to claim 27, wherein the chloride tobromide ratio is from about 30:70 to about 60:40.
 32. The methodaccording to claim 25, comprising sodium cations, calcium cations andbromide anions.
 33. The method according to claim 25, comprising sodiumcations, calcium cations and chloride anions.
 34. The method accordingto claim 19, wherein the viscosified composition comprises from about0.01 to about 45.0 percent polymer by weight, from about 10.0 to about90.0 percent water by weight; from about 0.05 to about 85.0 weightpercent of the first salt; and from about 0.05 to about 85.0 weightpercent of the second salt.
 35. The method according to claim 19,wherein the viscosified composition comprises from about 0.5 to about10.0 percent polymer by weight, from about 10.0 to about 90.0 percentwater by weight; from about 2 to about 80 weight percent of the firstsalt; and from about 2 to about 40 weight percent of the second salt.